1. Field of the Invention
This invention relates to optical coatings for infrared windows and domes, primarily for use on aircraft, and, more specifically to a polymer or plastic optical coating to protect infrared optics, particularly infrared window and domes.
2. Brief Description of the Prior Art
To increase the survivability and operational capability of infrared windows and domes, particularly as used during flight and particularly for the 8 to 12 micrometer wavelength region, protective coatings are required for rain, dust and sand and hail impact. The impact of these particles during flight (aircraft, missile, helicopter, etc.) on the window or dome erodes the window or dome, thereby reducing its strength and ability to transmit infrared therethrough. This degradation can render the electro-optical sensor behind the dome or window inoperable or even damaged should the window or dome catastrophically fail.
Presently used prior art infrared windows and domes degrade in performance due to loss of transmission and strength due to environmental degradation, particularly due to erosion by rain, dust and sand particles at aerodynamic speeds.
Prior art solutions to this problem have involved the use of a protective coating on the infrared domes and windows. Due to the requirement that the protective coating must be transparent in the wavelength region in which the window or dome operates (i.e., 8 to 12 micrometers, 3 to 12 micrometer, 3 to 5 micrometerns, 1 to 12 micrometer, etc.), past and current efforts on protective coatings have concentrated on traditional inorganic materials, such as silicon, gallium phosphide, boron phosphide, diamond, germanium carbide, silicon nitride, silicon carbide, oxides, etc. to obtain the desired transparency. These coating have displayed high strength, high fracture toughness, high hardness and moderate to high elastic (Young's) modulus.
The general mechanical requirements of coatings for soft (rain) and hard (sand, hail, dust, etc.) particle impact protection of substrates are low hardness and high fracture toughness or strength with a high elastic modulus to reduce the strain induced in the substrate or a low elastic modulus to absorb the impact stress. Accordingly, the above-mentioned materials have shown only limited effectiveness in solving the problem of erosion due to particle impact and have been difficult to scale up in size. It is therefore apparent that other solutions to the problem are required which overcome or minimize the problems presented by prior art solutions.